1. Field of the Invention
The present invention relates to a signal modulation circuit, and particularly relates to a circuit for performing delta sigma modulation.
2. Description of the Related Art
Conventionally, delta sigma modulation (ΔΣ modulation) is used in switching amplifiers or the like. A delta sigma modulator has a subtractor, an integrator, a quantizer, and a quantization error feedback circuit.
FIG. 4 illustrates a basic configuration of a delta sigma modulation circuit. A subtractor 16 calculates a difference between an input signal and a feedback signal, and an integrator 10 integrates the differential signal. An integrated signal is quantized by a quantizer 14 and is output as, for example, 1 bit (=binary) signal. The quantization error is fed back via a delay device 12.
JP 2007-312258 A discloses a delta sigma modulation circuit including an integrator group, an adder group, a quantizer, and a pulse width round-up circuit, and further discloses that a signal is converted into a one-bit signal synchronized with a sampling clock so as to be output. Further, this Patent Document discloses that a D-type flip-flop is used as a quantizer. JP 2012-527187 W also discloses a delta sigma modulation circuit.
Japanese Patent No. 4805177 describes a ternary one-bit digital amplifier that includes a switching unit for changing over a plurality of switches between on and off so as to provide a first state that a positive voltage is applied to a load, a second state that a voltage is not applied to the load, and a third state that a negative voltage is applied to the load.
In the configuration shown in FIG. 4, the delay device 12 is provided to a feedback path so that noise shaping is performed. At the same time, however, the configuration has a problem that the delay device 12 in the feedback path cannot correct an output state in real time, or a problem that noise shaping is not performed on distortion/noise components generated in the delay device and the distortion/noise components are directly output.
Further, when the delta sigma modulation circuit is used in a one-bit audio amplifier, pulse width modulation (PWM) and pulse density modulation (PDM) are used as a system for converting an input signal into a one-bit digital signal. When PDM that is suitable for a case where an input signal is expressed by pulse density and frequency is used, a zero level is inserted at predetermined timing so that a pulse width is maintained, and a level of the input signal is securely modulated into a frequency of the pulse.
In the Japanese Patent No. 4805177, a digital signal is generated by the delta sigma modulation and a ternary one-bit signal is generated, but an output signal from a switching circuit is fed back to the delta sigma modulation circuit, and a driver circuit is not taken into consideration.
It is an object of the present invention to provide a circuit for correcting an output state in real time, reducing influences of distortion/noise components generated by a delay device, and also reducing distortion of a driver circuit.